1. Field of the Invention
This invention relates to a calcination furnace, and more particularly to a furnace for calcining a ceramic product.
2. Description of the Relevant Art
In general, a ceramic product used for various kinds of electronic circuit elements is manufactured by mixing a ceramic powder with an organic binder to prepare an intermediate product and then subjecting it to calcination in a calcination furnace. The calcination furnaces include a continuous furnace for mass production and a batch furnace for mass production, as well as a test furnace and the like. Such a conventional calcination furnace is generally constructed so that a furnace chamber in which an intermediate product to be calcined (hereinafter referred to as "calcined intermediate product" or "intermediate product") is subjected to calcination is defined in a furnace body invariably or fixedly constructed. Thus, an internal volume of the furnace chamber is kept fixed.
This results in the conventional calcination furnace failing to vary a size of the furnace chamber to variably set conditions for calcination such as a calcination temperature, a temperature distribution, a flow rate of gas and the like depending on a calcined intermediate product.
In the conventional calcination furnace, a heater utilizing combustion of fuel such as oil or gas (hereinafter referred to as "combustion heater") and an electric heater are used as a heat source for calcination. The combustion heater is decreased in running cost as compared with the electric heater. The combustion heater generally employs a direct heating system of carrying out combustion of fuel in the furnace chamber in which a calcined intermediate product is placed when the intermediate product is not affected by combustion gas produced due to combustion of fuel, whereas it employs an indirect heating system when the calcined intermediate product is adversely affected by the combustion gas.
The indirect heating system is classified into a radiant tube system and a muffle construction system. The radiant tube system is constructed so as to burn fuel in a plurality of heat-resistant cylinders or tubes. The heat-resistant tubes each are formed of a metal or ceramic material and inserted through the furnace body into the furnace chamber. The muffle construction system has a muffle furnace body formed of a heat-resistant metal or refractory material so as to define the furnace chamber or calcination chamber and introduce combustion gas produced in a combustion furnace provided separate from the muffle furnace body into the calcination chamber to heat an intermediate product placed in the calcination chamber.
Unfortunately, the radiant tube system requires to arrange a number of radiant tubes to cause an extensive increase in cost of equipment. Also, it has another disadvantage of failing to provide uniform heat transfer, even when the radiant tubes are arranged at an increased cost described above in such a manner that a distance between a wall surface of the furnace chamber and the radiant tubes exceeds a diameter of the radiant tube. In the muffle construction system, when the muffle furnace body is made of a heat-resistant metal material, a service temperature of the furnace body is limited to 900.degree. C. or below in view of its heat-resistance. The calcination at a higher temperature requires that the furnace body is formed of refractory bricks into an arch-like shape, resulting in being labor- and time-consuming and increased in cost.
Further, when the combustion heater is used as the heat source, a heat exchange system is generally employed. The heat exchange system uses a heat exchanger as a means for improving heat efficiency, so that heat exchange is carried out between combustion gas produced due to combustion of fuel and combustion air fed to the combustion heater for combustion to preheat the combustion air, followed by feeding the preheated combustion air to a burner constituting the combustion heater. The conventional heat exchange system is so constructed that the heat exchanger is arranged outside the calcination furnace and the heat exchanger and calcination furnace are connected to each other through pipings. Unfortunately, connection between the heat exchanger arranged separate from the calcination furnace and the calcination furnace through the pipings causes the pipings to be complicated in construction. Also, this deteriorates heat efficiency due to discharge of heat from the pipings.
Further, in view of an operation manner of the calcination furnace, the calcination furnace is classified into a continuous furnace and a batch furnace. The continuous calcination furnace is mainly used for mass production, whereas the batch calcination furnace is used for mass production, as well as used as a test furnace for determining calcination conditions for the continuous calcination furnace. The continuous calcination furnace is so operated that a plurality of calcined intermediate products are charged in a furnace chamber and calcined while being transferred from one side of the furnace to the other side thereof. Therefore, the continuous furnace causes a variation in temperature to occur between the intermediate products being transferred in the furnace chamber. On the contrary, the batch furnace is adapted to calcine the intermediate products in a furnace chamber while keeping them stationary therein, to thereby prevent occurrence of such a variation in temperature between the intermediate products as encountered with the continuous furnace. Also, the batch furnace is constructed so as to minimize a variation in temperature in the furnace chamber.
Thus, it is impossible to correspond the continuous furnace and batch furnace to each other. For example, calcination conditions determined for the continuous furnace are not applied to the batch furnace; whereas use of the batch furnace as a test furnace for determining calcination conditions for the continuous furnace fails to provide accurate calcination conditions for the continuous furnace.
Controlling of an atmosphere in the calcination furnace is carried out in a manner to continuously feed fresh gas for the atmosphere to the furnace chamber while exhausting the atmosphere. The fresh gas is introduced into the furnace chamber at a temperature as close as a room temperature, resulting in the atmosphere in the furnace chamber being locally cooled to adversely affect calcination conditions, leading to a variation in quality or characteristics of final products which have been subjected to calcination (hereinafter referred to "final calcined products"). Also the exhaust gas is discarded regardless of a large amount of thermal energy contained therein.